The auxiliary or supplemental oxygen supply in commercial aircraft, in particular, after losing the integrity of the pressurized cabin is generally realized with Constant-Flow or Demand systems. While the latter are reserved for the cockpit crew—and usually have an overpressure function and a Quick-Donning design—only Constant-Flow masks according to TSO64, (TSO-C64a)—that typically comprise a reservoir bag—have been used so far in the cabin area.
One decisive disadvantage of the Constant-Flow concept is the lack of controllability. For the range above 18 kft (18 000 feet), the controller or regulator provides a respiratory volume per minute of 30 l/min as worst-case scenario, wherein this respiratory volume may, at worst, occur over a duration of a few minutes for a few passengers and stands in contrast to a typical value of 6-8 l/min. Since the extreme value may occur at any respiratory connection, which means the outlet to the passenger, all connections need to be operated with an increased oxygen dosage that results in oxygen losses of 30-70% in comparison with an optimized supply. On the other hand, the low technical complexity of this system is exceptionally advantageous and cost-efficient, wherein the utilization of a Demand system as it is used in the cockpit area cannot be considered for numerous reasons. One possible alternative that has been offered by several manufacturers for a few years is the so-called pulse-dosed system, in which a brief oxygen blast is introduced into the respective respiratory connection via a correspondingly controlled valve or control valve when the beginning of the inhalation phase is detected. This makes it possible to individually adapt the demand at each respiration point—the metering takes place in accordance with the breathing rate—and the pulse width and therefore the admixed quantity per breath can be controlled in dependence on the altitude. Due to the introduction of the oxygen in the first stage of the inhalation phase, the oxygen reaches the deep airways, which form the lower portions of the lungs (alveoli) where the actual gas exchange takes place; the last stage of the inhalation phase only reaches the supplying or conducting airways, in which the oxygen supplied with other systems cannot be utilized. One certain disadvantage of this very economical system is the frequently unreliable detection of the respiratory activity, e.g., if the mask leaks; this results in oxygen only being admixed during a few breaths or, in extreme instances, not at all. Between the beginning of the inhalation phase and the arrival of the oxygen dose in the upper airways, a certain delay time elapses that contradicts the ideal function of the system; in addition, one has to consider the insufficient fulfillment of the requirement for making available pure oxygen in the maximum altitude range beginning at approximately 35 kft (35 000 feet) such that the approval of this system, in particular, above 40 kft (40 000 feet) is highly questionable.